1. Field of the Invention
The present invention relates to an electrophotographic apparatus capable of optically scanning and duplicating an original document (e.g., "photocopier"). In particular, the present invention relates to an electrophotographic copying apparatus having substantially increased copying speed and copy output, with minimal increase in electrostatic toner consumption, and with minimal degradation in copy quality.
2. Description of the Prior Art
Electromechanical devices capable of electrophotographically duplicating printed documents, commonly known as "photocopiers," are well known in the art. Such a device comprises two basic functional assemblies: an image generator assembly and an image transfer assembly.
The image generator assembly typically consists of an electromechanical scanning assembly containing an electrical lamp and a mirror. This assembly scans the original document illuminating it with the electrical lamp. Light reflected from the original document, representative of the image thereon, is reflected via the mirror to the image transfer assembly. By scanning the full width and length of the original document, a full image of the document as contained in the reflected light is transmitted via the mirror to the image transfer assembly.
The image transfer assembly contains a cylindrical drum having a photosensitive outer surface which is electrostatically charged by an electrostatic generator. The reflected light containing the image information of the original document received from the image generator assembly is used to expose the outer surface of this photosensitive drum as it rotates. This causes a latent electrostatic image to form on the outer surface of the drum which corresponds to and is the negative of the original document image. Electrostatic toner particles are then applied to the electrostatically charged surface of the drum. Black image areas, i.e., areas of the electrostatic image corresponding to areas of the original document containing image information, attract and retain the electrostatic toner particles. White image areas, i.e., areas of the latent electrostatic image corresponding to areas of the original document containing no image information, do not attract and therefore do not retain any electrostatic toner particles.
The rotating electrostatically charged surface of the drum, now containing areas retaining electrostatic toner particles, is brought into physical contact with the surface of a sheet of paper moving with a linear velocity substantially equal to the tangential velocity of the rotating drum. Simultaneous with this contact, an electrostatic charge opposite in potential to that originally applied to the outer surface of the rotating drum is generated at and applied to the opposite surface of the paper. This opposite electrostatic charge attracts the electrostatic toner particles, drawing them away from the outer surface of the rotating drum into the surface of the paper.
The paper, now having electrostatic toner particles impressed thereon and forming an image duplicating that of the original document, then passes through a series of heated rollers. This heat and pressure from the rollers causes the electrostatic toner particles to permanently bond to the surface of the paper. Thus, a substantially identical copy of an original document is created.
More detailed description of this typical, well-known photocopying process may be found in Komori et al., U.S. Pat. No. 4,411,514 and McGraw-Hill Encyclopedia of Science and Technology, Vol. 13, pages 373-75 (6th ed., 1989).
Implicit in this basic photocopying process is the requirement that the tangential speed of the outer surface of the rotating photosensitive drum, and therefore its rotational speed, corresponds to the speed of the scanning assembly scanning the original document. With proper correspondence of these speeds, the electrostatic image formed on the photosensitive drum and therefore transferred to the paper will be a substantially true copy of the original document, i.e., 1:1 original-to-copy image correspondence in both dimensions.
Komori et al., U.S. Pat. No. 4,411,514 discloses a photocopying apparatus which provides for varying the rotational speed of the rotating photosensitive drum relative to the linear speed of the scanning assembly, thereby allowing magnified or reduced photocopies to be made. According to the teachings of Komori et al., as the rotational speed of the rotating photosensitive drum is increased or decreased, the quantities of electrostatic charge and electrostatic toner applications are increased or decreased proportionally, respectively. Thus, as the rotational speed of the rotating photosensitive drum varies, the copy image quality remains substantially consistent.